1. Field of the Invention
The invention relates to a method for the double-side grinding of semiconductor wafers, in particular, to a method for the alignment of double-side grinding machines through improved orientation of the grinding spindles of double-side grinding machines, correction of the grinding spindle positions, and suitable devices for carrying out the method.
2. Background Art
Double-side grinding machines are used in mechanical machining steps in fabrication sequences of the wafer industry for producing semiconductor wafers, in particular silicon wafers. A mechanically abrasive, material-removing machining of the semiconductor wafers is involved.
Simultaneous double-side grinding (“double disk grinding”, DDG) is often used in order to achieve a particularly good geometry of the machined semiconductor wafers, in particular in comparison with alternative machining methods such as so-called lapping methods. A suitable DDG method and devices suitable for carrying out the method are known, for example from EP 868 974 A2.
The semiconductor wafer is machined simultaneously on both sides in free-floating fashion between two grinding wheels or disks mounted on opposite spindles. In this case, the semiconductor wafer is guided in a manner substantially free of constraint forces axially between two water or air pads (e.g. the so-called hydropads) and prevented from “floating away” radially by a guide ring or by individual radial spokes. During the grinding process, the semiconductor wafer is rotated, usually in a manner driven by a so-called “notch finger” that engages into the orientation notch of the semiconductor wafer.
Suitable DDG machines are offered for example by Koyo Machine Industries Co., Ltd. The model DXSG320 is suitable for grinding semiconductor wafers having a diameter of 300 mm. Diamond grinding disks are usually used as grinding tools.
What is particularly critical in the DDG method is the orientation of the two grinding spindles (=shafts) on which the grinding disks are mounted. The two spindles should be oriented exactly collinearly in the course of the basic setting of the machine since deviations (radially, axially) adversely influence the shape and nanotopology of the wafer. As far as the shape of the wafer is concerned, terms employed by the person skilled in the art to characterize the shape include bow and warp.
Proceeding from this (often asymmetrical) basic setting, the spindles are subsequently tilted symmetrically in order to satisfy corresponding product criteria, inter alia with regard to the grinding pattern (cross-grinding) or the global geometry GBIR (formerly: TTV, “total thickness variation”). JP 2001-062718 discloses a corresponding method. With an already equipped machine in the working position, the offset of the wafer perpendicular to the spindle direction (radially) is measured by means of eddy current sensors and the position of the grinding spindles is set accordingly. The grinding spindles are thus moved with the grinding disks fixed on them in the working position and tilted essentially symmetrically with respect to the basic setting (tilt or grinding tilt).
In the context of this invention, the asymmetrical deviations of the axial alignment are also referred to as parallelism deviation or angular deviation. The terms machine axial alignment or simply axial alignment are also familiar to the person skilled in the art in this connection. Parallelism deviation is intended to denote the distance between the center lines of the two grinding spindles at a specific point, and angular deviation the angle between these two center lines.
In the prior art efforts have already been made to solve the problems outlined since—as mentioned above—grinding spindles that are not oriented exactly in the basic setting have a considerable influence on the grinding result.
EP 1 616 662 A1 describes a method which provides for determining, in the working position, in each case the distances between the hydropads and three predetermined positions on the front and rear sides of the workpiece by means of displacement sensors, for calculating therefrom deformations of the workpiece with respect to the at least three positions, and for correspondingly orienting the axial positions of the grinding disks in the event of excessively large deviations.
DE 10 2004 011 996 A1 likewise discloses integrating into hydropads one or a plurality of measuring sensors which, during the grinding process, make it possible to measure the distance between the surface of the hydropads and the workpiece surface. These distance measurements serve for centering the workpiece between the hydropads by means of axial displacement of the grinding spindles in such a way that the distance between the workpiece and the hydropad becomes identical on both sides of the workpiece. A similar method, which refers in particular to a center plane of the workpiece and provides three distance sensors in the wafer guide, is also known from DE 10 2004 053 308 A1.
What is disadvantageous about the known methods is that the parallelism deviation of the grinding spindles (distance between the center lines of the spindles) is left out of consideration for lack of radial measured values. The basic setting of the grinding spindles cannot be corrected by the methods described. This also applies to the method disclosed in JP 2001-062718.
For carrying out the distance measurement itself, mechanical probes, as disclosed e.g. in JP 2005-201862, and eddy current sensors are known. Furthermore, optical measuring units, e.g. by means of a laser, are already prior art. Measuring units of this type are available e.g. from db Prüftechnik (OPTALIGN® models). Commercially available inclinometers (electrical spirit levels) are suitable for the angle measurement.